The present invention relates to an overload prevention apparatus preferably employable for a hoist or similar device mounted on crane or similar machine and, more particularly, to an apparatus for preventing an damage or distortion of a crane or similar machine which occurs when a heavier load than a rated load is hoisted.
If often happens with container cranes, that a platform or pallet which is being hoisted may snag the hatch of a vessel, so that the container is suddenly held back by the vessel, or similar malfunctions may occur. Once such trouble has occurred, the crane or a part of the vessel is likely to be damaged. To prevent this, a few proposals have already been made with respect to an overload prevention apparatus to be mounted on a crane to prevent damage deformation distortion to the crane.
Typical conventional overload prevention apparatuses disclosed in for example Japanese Patent Publication No. 51878/1983 (corresponding to GB 1538555 and DE-OS 2610267) and the force exerted on ropes by means of which a hoisting platform or the like is suspended from the hoist is detected directly or indirectly and, when it is found that a load heavier than the rated load is carried thereon, the hoisting is operation stopped.
In Japanese Patent Publication No. 51878/1983 another conventional apparatus is proposed wherein the ropes are resiliently supported while they are stretched radially by clamps and the supporting section is provided with a limit switch for detecting an overload by the displacement of the clamps.
In, for example, Japanese Patent Laid-Open No. 125597/1983, an excessive load prevention apparatus includes an current detector mounted on a motor for the hoist so as to detect an overload by the detected value of the current.
In the proposed overload prevention apparatus an overload is detected by a limit switch, a current detector or a similar instrument, the hoist motor is turned off and the brake is actuated. Although the hoist attempts to stop, it often happens that the drum continues to rotate due to inertia of the motor and the speed reduction mechanism, so that the hoist continues to lift the load. The larger the crane, the more likely this is to occur. With the conventional crane hoist, the crane must be halted frequently to prevent damage when an overload is detected. This is a critical problem when a pallet or the like means suddenly snags the rim of a hatch or some other obstruction during high-speed hoisting. Such accidents almost always damage the crane.
Thus, the present invention has been made with the foregoing background in mind and its object resides in providing an improved smaller, more reliable overload prevention apparatus which is not likely to excessively increase rope tension.
To accomplish the above object according to the present invention an apparatus for preventing an overload on a hoist or the like is proposed, wherein the apparatus includes a geared member including a number of teeth comprising valleys and peaks arranged alternately one after another, with an engagement member including an engagement portion dimensioned so as to allow the latter to enter the valley between the adjacent teeth on the geared member, and a supporting member for turnably supporting the engagement member so as to allow the latter to turn toward and away from the geared member. A resilient member is disposed between the engagement member and the supporting member to impel the engagement portion on the engagement member toward the geared member, with either the geared member or the supporting member being connectable to a hoisting platform or the like and the one member being arranged to move relative to the other in the direction of extension of the teeth. The engagement member and the teeth are arranged such that the engagement member is displaced away from the valley as the movable member is displaced.
Other objects, features and advantages of the present invention will become more clearly apparent from reading the following description which has been prepared in conjunction with the accompanying drawings.